The acetylcholine-activated K+ channel (IK,Ach) is a member of the superfamily of proteins known as the G-protein-coupled inward rectifier K+ (GIRK) channels. GIRK channels function as cellular mediators of a wide range of hormones and neurotransmitters and are expressed in cardiac muscle, skeletal muscle, neurons, and endocrine tissue. The cardiac IK,Ach is composed of the inward rectifier K+ (Kir) channel subunits Kir3.1 and Kir3.4 (GIRK1/4), which are arranged in a tetramer. In the heart, binding of muscarinic agents such as acetylcholine or carbachol to the muscarinic (M2) receptor causes a dissociation of the βγ subunits of the G inhibitory protein (Gi), which subsequently bind to and activate IK,Ach. Once opened, IK,Ach allows the movement of K+ out of the cardiac myocyte, causing the cell membrane potential to become more negative.
Cardiac arrhythmias are defined as abnormalities in the generation or conduction of electrical impulses in the heart. Atrial fibrillation (AF) is a rapid and irregular atrial muscle arrhythmia that results in erratic and incomplete cardiac contractions. AF is the most prevalent arrhythmia in the United States, affecting more than 2 million adult patients. The frequency of AF increases with age, occurring in 3% to 5% of those older than 65 years of age and 9% of people older than 80 years. Regardless of the underlying cardiac disease, chronic AF is associated with increased morbidity and mortality. Current antiarrhythmic drugs used in the treatment of AF are limited by suboptimal efficacy and a high incidence of toxicity. One novel target for AF drug therapy is the GIRK1/4 channel. Recent studies indicate that IK,Ach is constitutively active in patients with AF. This constitutively active channel causes the atrial action potential duration to shorten with a resulting increase in cell excitability. Therefore, GIRK1/4 channel blockers, by decreasing atrial excitability, should reduce the incidence of AF. However, discovery of new drugs that bind to and block the cardiac IK,Ach has been hampered by the absence of a cell-based screening assay that uses the GIRK1/4 channel.
As such, methods to determine new drugs that bind to and inhibit IK,Ach would be desirable. An assay system that is capable of detecting such new drugs would be particularly beneficial.